Coupling optical signals into silicon optoelectronic chips

ABSTRACT

A method and system for coupling optical signals into silicon optoelectronic chips are disclosed and may include coupling one or more optical signals into a back surface of a silicon photonic chip through a light path in a region where silicon is removed from said silicon photonic chip, wherein photonic devices may be integrated in layers on a front surface of the silicon photonic chip. Optical couplers, such as grating couplers, may receive the optical signals in the front surface. The optical signals may be coupled into the back surface of the chips via optical fibers and/or optical source assemblies. The region where silicon may be removed from said silicon photonic chip may comprise silicon dioxide. The chip may be bonded to a second chip. Optical signals may be reflected back to the optical couplers via metal reflectors, which may be integrated in dielectric layers on the chips.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application makes reference to and claims priority to U.S.Provisional Application Ser. No. 61/198,660 filed on Nov. 6, 2008, whichis hereby incorporated herein by reference in its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[Not Applicable]

MICROFICHE/COPYRIGHT REFERENCE

[Not Applicable]

FIELD OF THE INVENTION

Certain embodiments of the invention relate to signal processing. Morespecifically, certain embodiments of the invention relate to a methodand system for coupling optical signals into silicon optoelectronicchips.

BACKGROUND OF THE INVENTION

As data networks scale to meet ever-increasing bandwidth requirements,the shortcomings of copper data channels are becoming apparent. Signalattenuation and crosstalk due to radiated electromagnetic energy are themain impediments encountered by designers of such systems. They can bemitigated to some extent with equalization, coding, and shielding, butthese techniques require considerable power, complexity, and cable bulkpenalties while offering only modest improvements in reach and verylimited scalability. Free of such channel limitations, opticalcommunication has been recognized as the successor to copper links.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present invention as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

A system and/or method for integrated control system for couplingoptical signals into silicon optoelectronic chips, substantially asshown in and/or described in connection with at least one of thefigures, as set forth more completely in the claims.

Various advantages, aspects and novel features of the present invention,as well as details of an illustrated embodiment thereof, will be morefully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram of a photonically enabled CMOS chip, inaccordance with an embodiment of the invention.

FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordancewith an embodiment of the invention.

FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to anoptical fiber cable, in accordance with an embodiment of the invention.

FIG. 2 is a block diagram of an exemplary grating coupler structure, inaccordance with an embodiment of the invention.

FIG. 3 is a schematic of an exemplary optical fiber coupled to the backsurface of a CMOS photonic chip, in accordance with an embodiment of theinvention.

FIG. 4 is a schematic of an exemplary optical fiber coupled to the backsurface of a CMOS photonic chip with an etched substrate light path, inaccordance with an embodiment of the invention.

FIG. 5 is a schematic of an exemplary optical fiber coupled to the backsurface of a CMOS photonic chip with an etched substrate light path andmetal reflector, in accordance with an embodiment of the invention.

FIG. 6 is a schematic of an exemplary optical fiber coupled to the backsurface of a CMOS photonic chip flip-chip bonded to a packagingsubstrate, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain aspects of the invention may be found in a method and system forcoupling optical signals into silicon optoelectronic chips. Exemplaryaspects of the invention may comprise coupling one or more opticalsignals into a back surface of a CMOS photonic chip comprising photonic,electronic, and optoelectronic devices. The photonic, electronic, andoptoelectronic devices may be integrated in a front surface of the CMOSphotonic chip, and one or more grating couplers may receive the one ormore optical signals in the front surface of the CMOS photonic chip. Theone or more optical signals may be coupled into the back surface of theCMOS photonic chip via one or more optical fibers, which may be affixedto the CMOS photonic chip utilizing epoxy. The one or more opticalsignals may be coupled into the back surface of the CMOS photonic chipvia one or more optical source assemblies, which may be affixed to theCMOS photonic chip via epoxy. The optical signals may be coupled to theone or more grating couplers via a light path etched in the CMOSphotonic chip. The etched light path may be refilled with silicondioxide. The CMOS photonic chip may be flip-chip bonded to a packagingsubstrate. Optical signals that pass through the one or more gratingcouplers may be reflected back to the one or more grating couplers viaone or more metal reflectors, which may be integrated in dielectriclayers on the CMOS photonic chip.

FIG. 1A is a block diagram of a photonically enabled CMOS chip, inaccordance with an embodiment of the invention. Referring to FIG. 1A,there is shown optoelectronic devices on a CMOS chip 130 comprisingoptical modulators 105A-105D, high-speed photodiodes 111A-111D, monitorphotodiodes 113A-113H, and optical devices comprising taps 103A-103K,optical terminations 115A-115D, and grating couplers 117A-117H. There isalso shown electrical devices and circuits comprising transimpedance andlimiting amplifiers (TIA/LAs) 107A-107E, analog and digital controlcircuits 109, and control sections 112A-112D. Optical signals arecommunicated between optical and optoelectronic devices via opticalwaveguides fabricated in the CMOS chip 130.

The optical modulators 105A-105D comprise Mach-Zehnder or ringmodulators, for example, and enable the modulation of the CW laser inputsignal. The optical modulators 105A-105D comprise high-speed andlow-speed phase modulation sections and are controlled by the controlsections 112A-112D. The high-speed phase modulation section of theoptical modulators 105A-105D may modulate a CW light source signal witha data signal. The low-speed phase modulation section of the opticalmodulators 105A-105D may compensate for slowly varying phase factorssuch as those induced by mismatch between the waveguides, waveguidetemperature, or waveguide stress and is referred to as the passivephase, or the passive biasing of the MZI.

This mismatch between the waveguides may be intentional, such as in animbalanced MZI, but is often unintentional in a nominally balanced MZIdue to small imperfections in waveguide fabrication. The phasemodulators then have a dual role: to compensate for the passive biasingof the MZI and to apply the additional phase modulation used to modulatethe light intensity at the output port of the MZI according to a datastream. The former phase tuning and the latter phase modulation may beapplied by separate, specialized devices, since the former is a lowspeed, slowly varying contribution, while the latter is typically a highspeed signal. These devices are then respectively referred to as theLSPM and the HSPM. Examples for LSPM are thermal phase modulators (TPM),where a waveguide portion is locally heated up to modify the index ofrefraction of its constituting materials, or forward biased PINjunctions (PINPM) where current injection into the PIN junction modifiesthe carrier density, and thus the index of refraction of thesemiconductor material. An example of HSPM is a reversed biased PINjunction, where the index of refraction is also modulated via thecarrier density, but which allows much faster operation, albeit at alower phase modulation efficiency per waveguide length.

The outputs of the modulators are optically coupled via waveguides tothe grating couplers 117E-117H. The taps 103D-103K comprise four-portoptical couplers, for example, and are utilized to sample the opticalsignals generated by the optical modulators 105A-105D, with the sampledsignals being measured by the monitor photodiodes 113A-113H. The unusedbranches of the taps 103D-103K are terminated by optical terminations115A-115D to avoid back reflections of unwanted signals.

The grating couplers 117A-117H comprise optical gratings that enablecoupling of light into and out of the CMOS chip 130. The gratingcouplers 117A-117D are utilized to couple light received from opticalfibers into the CMOS chip 130, and the grating couplers 117E-117H areutilized to couple light from the CMOS chip 130 into optical fibers. Theoptical fibers may be epoxied, for example, to the CMOS chip, and may bealigned at an angle from normal to the surface of the CMOS chip 130 tooptimize coupling efficiency. In an embodiment of the invention, theoptical fibers and a laser source module that generates the CW Laser In101 signal may be coupled to the back surface of the CMOS chip. In thismanner, the optical interface may be located on the back surface of theCMOS chip 130, and the electrical and thermal interfaces may be locatedon the front surface of the CMOS chip 130.

The high-speed photodiodes 111A-111D convert optical signals receivedfrom the grating couplers 117A-117D into electrical signals that arecommunicated to the TIA/LAs 107A-107D for processing. The analog anddigital control circuits 109 may control gain levels or other parametersin the operation of the TIA/LAs 107A-107D. The TIA/LAs 107A-107D thencommunicate electrical signals off the CMOS chip 130.

The control sections 112A-112D comprise electronic circuitry that enablemodulation of the CW laser signal received from the splitters 103A-103C.The optical modulators 105A-105D require high-speed electrical signalsto modulate the refractive index in respective branches of aMach-Zehnder interferometer (MZI), for example.

In operation, a laser source coupled to the back surface of the CMOSchip 130 may generate a CW light signal, CW Laser In 101, which may betransmitted through the substrate, or through an etched region of thesubstrate, to a grating coupler on the front surface of the CMOS chip130. The received optical signal may then be processed by the opticalmodulators 105A-105D, for example. Similarly, optical fibers may beaffixed to the back surface of the CMOS chip 130, using epoxy, forexample. Optical signals may then be communicated from the opticalfibers, through the back surface of the CMOS chip 130 and to one or moregrating couplers on the front surface of the CMOS chip 130. By couplingoptical sources to the back surface of the CMOS chip 130, the chip maythen be flip-chip bonded to a package. In this manner, the back surfaceof the CMOS chip 130 comprises an optical interface, and the frontsurface comprises an electrical and thermal interface.

FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordancewith an embodiment of the invention. Referring to FIG. 1B, there isshown the CMOS chip 130 comprising electronic devices/circuits 131,optical and optoelectronic devices 133, a light source interface 135,CMOS chip front surface 137A, CMOS chip back surface 137B, an opticalfiber interface 139, and CMOS guard ring 141.

The light source interface 135 and the optical fiber interface 139comprise grating couplers that enable coupling of light signals via theCMOS chip surface 137, as opposed to the edges of the chip as withconventional edge-emitting devices. Coupling light signals via the CMOSchip surface 137 enables the use of the CMOS guard ring 141 whichprotects the chip mechanically and prevents the entry of contaminantsvia the chip edge.

The electronic devices/circuits 131 comprise circuitry such as theTIA/LAs 107A-107D and the analog and digital control circuits 109described with respect to FIG. 1A, for example. The optical andoptoelectronic devices 133 comprise devices such as the taps 103A-103K,optical terminations 115A-115D, grating couplers 117A-117H, opticalmodulators 105A-105D, high-speed photodiodes 111A-111D, and monitorphotodiodes 113A-113H.

In an embodiment of the invention, optical sources, such as a lasersource assembly and optical fibers, may be coupled to the CMOS chip backsurface 137B, whereas electrical, optical, and optoelectronic devicesmay be integrated in or on the CMOS chip front surface 137A. By couplingoptical sources to the back surface of the CMOS chip 130, the chip maythen be flip-chip bonded to a package. In this manner, the back surfaceof the CMOS chip 130 comprises an optical interface, and the frontsurface comprises an electrical and thermal interface.

FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to anoptical fiber cable, in accordance with an embodiment of the invention.Referring to FIG. 1C, there is shown the CMOS chip 130 comprising theCMOS chip front surface 137A, the CMOS chip back surface 137B, and theCMOS guard ring 141. There is also shown a fiber-to-chip coupler 143, anoptical fiber cable 145, and an optical source assembly 147.

The CMOS chip 130 comprising the electronic devices/circuits 131, theoptical and optoelectronic devices 133, the light source interface 135,the CMOS chip surface 137, and the CMOS guard ring 141 may be asdescribed with respect to FIG. 1B.

In an embodiment of the invention, the optical fiber cable may beaffixed, via epoxy for example, to the CMOS chip back surface 137B. Thefiber chip coupler 143 enables the physical coupling of the opticalfiber cable 145 to the CMOS chip 130.

Similarly, the optical source assembly 147 may be affixed, via epoxy orsolder, for example, to the CMOS chip back surface 137B. In this mannera high power light source may be integrated with optoelectronic andelectronic functionalities of one or more high-speed optoelectronictransceivers on a single CMOS chip.

By coupling optical sources to the back surface of the CMOS chip 130,the chip may then be flip-chip bonded to a package. In this manner, theback surface of the CMOS chip 130 comprises an optical interface, andthe front surface comprises an electrical and thermal interface.

FIG. 2 is a block diagram of an exemplary grating coupler structure, inaccordance with an embodiment of the invention. Referring to FIG. 2,there is the CMOS chip 130 comprising a silicon substrate 201, a buriedoxide 203, a silicon waveguide 205, and a grating coupler 207.

The silicon substrate 201 comprises a CMOS processed wafer, such as asilicon-on-insulator (SOI) substrate, with electronic, photonic, andoptoelectronic devices, such as the grating coupler 207, which may befabricated etched into a silicon layer deposited on the buried oxide203. In another embodiment of the invention, the silicon layer and theburied oxide may be generated by oxygen ion implantation, therebygenerating the buried oxide 203 under the silicon layer that may befabricated into the silicon waveguide 205 and the grating coupler 207.

In an embodiment of the invention, an optical source, such as a lasersource assembly or one or more optical fibers, may be coupled to theback surface of the silicon substrate 201. The optical signal may travelthrough the silicon substrate to the grating coupler 207 near the frontsurface, thereby introducing the optical power into the siliconwaveguide 205. Back-side coupling is illustrated further in FIGS. 3-6.

FIG. 3 is a schematic of an exemplary optical fiber coupled to the backsurface of a CMOS photonic chip, in accordance with an embodiment of theinvention. Referring to FIG. 3, there is shown the CMOS chip 130comprising a silicon substrate 301, a buried oxide 303, a silicon layer305, dielectric layers 307, metal pads 309, an optical fiber 311, epoxy313, and an anti-reflection coating 315. There is also shown the CMOSchip front surface 137A and the CMOS chip back surface 137B.

The silicon substrate 301, the buried oxide 303, the optical fiber 311,and the grating coupler 317 may be substantially similar to the siliconsubstrate 201, the optical fiber cable 145, and the grating coupler 207described with respect to FIGS. 1B-2. The silicon layer 305 may comprisea deposited silicon layer that may support optical, electrical, andoptoelectronic devices, such as the grating coupler 317. In anotherembodiment of the invention, the silicon layer 305 may be formed fromthe silicon substrate 301 by oxygen implantation forming the buriedoxide 303 below the silicon layer 305. The silicon substrate 301 may bethinned for reduced spacing between the fiber 311 and the gratingcoupler 317.

The dielectric layers 307 may comprise the back-end dielectric layers ofa CMOS process utilized to fabricate electrical, optical, andoptoelectronic devices in the silicon substrate 301. The dielectriclayers may be configured to provide a reflective surface for the lightpath of the optical signal from the fiber 311. In another embodiment ofthe invention, metal layers may be incorporated in, on, and/or under thedielectric layers 307 to provide a high reflectivity surface. In thismanner, a higher coupling efficiency may be obtained by the gratingcoupler 317. This is shown further with respect to FIG. 4.

The metal pads 309 comprise metal layers for electrical contact toelectronic and optoelectronic devices fabricated in the siliconsubstrate 301. In another embodiment of the invention, the metal pads309 comprise flip-chip bump bonds for coupling the CMOS chip 130 to apackage, as shown in FIG. 6.

The anti-reflection coating 315 comprises one or more dielectric layerswith thicknesses and dielectric constants configured to result in a lowreflectivity at the CMOS chip back surface 137B, thereby minimizingoptical losses.

In operation, an optical signal may be communicated from the fiber 311to the grating coupler 317 through the silicon substrate and the buriedoxide 303. The size, spacing, and orientation of the gratings in thegrating coupler 317 may be configured to generate an optical signal inthe plane of the silicon layer 305 from the optical signal received fromthe fiber 311. By coupling optical input and output structures, such asoptical fibers and light source assemblies, on the CMOS chip backsurface 137B, greater flexibility is enabled for locating the electricaland thermal interfaces on the CMOS chip front surface. For example, theCMOS chip 130 may then be flip-chip bonded to IC packages or otherchips. In addition, optical losses from the dielectric layers 307,comprising the CMOS process back-end layers, may be eliminated.

The reflectivity of the interface between the silicon layer 305 and theburied oxide 303 may be up to 20%, so it would be advantageous to removethis interface from the light path, as shown in FIGS. 4-6.

FIG. 4 is a schematic of an exemplary optical fiber coupled to the backsurface of a CMOS photonic chip with an etched substrate light path, inaccordance with an embodiment of the invention. Referring to FIG. 4,there is shown the CMOS chip 130 comprising the silicon substrate 301,the buried oxide 303, the silicon layer 305, the dielectric layers 307,the metal pads 309, the optical fiber 311, epoxy 313, theanti-reflection coating 315, the grating coupler 317, and a silicondioxide refill layer 401. There is also shown the CMOS chip frontsurface 137A and the CMOS chip back surface 137B.

The silicon substrate 301 may be etched such that the optical path of anoptical signal from the fiber 311 may avoid the interface between thesilicon substrate 301 and the buried oxide 303. The etched region maythen be backfilled with the silicon dioxide refill layer 401, therebyeliminating one silicon/silicon dioxide interface in the light path.

In operation, an optical signal may be communicated from the fiber 311to the grating coupler 317 through the silicon dioxide refill layer 401and the buried oxide 303. The size, spacing, and orientation of thegratings in the grating coupler 317 may be configured to generate anoptical signal in the plane of the silicon layer 305 from the opticalsignal received from the fiber 311. By coupling optical input and outputstructures, such as optical fibers and light source assemblies, on theCMOS chip back surface 137B, greater flexibility is enabled forelectrical and thermal interfaces on the CMOS chip front surface 137A.For example, the CMOS chip 130 may then be flip-chip bonded to ICpackages or other chips via the CMOS chip front surface 137A.

FIG. 5 is a schematic of an exemplary optical fiber coupled to the backsurface of a CMOS photonic chip with an etched substrate light path andmetal reflector, in accordance with an embodiment of the invention.Referring to FIG. 5, there is shown the CMOS chip 130 comprising thesilicon substrate 301, the buried oxide 303, the silicon layer 305, thedielectric layers 307, the metal pads 309, the optical fiber 311, epoxy313, the anti-reflection coating 315, the grating coupler 317, thesilicon dioxide refill layer 401, and a metal reflector 501. There isalso shown the CMOS chip front surface 137A and the CMOS chip backsurface 137B.

The silicon substrate 301 may be etched such that the optical path of anoptical signal from the fiber 311 may avoid the interface between thesilicon substrate 301 and the buried oxide 303. The etched region maythen be backfilled with the silicon dioxide refill layer 401, therebyeliminating one silicon/silicon dioxide interface in the light path. Inaddition, the metal reflector 501 may be integrated in the dielectriclayers 307, thereby integrating a highly reflective surface forreflecting any optical signal that passes through the grating coupler317 and enabling a second pass-through of the light signal through thecoupler. In this manner, a higher coupling efficiency may be obtained.The metal reflector 501 may comprise aluminum, copper, titanium,platinum, or other metal, for example.

In operation, an optical signal may be communicated from the fiber 311to the grating coupler 317 through the silicon dioxide refill layer 401and the buried oxide 303. The size, spacing, and orientation of thegratings in the grating coupler 317 may be configured to transferoptical signal into the plane of the silicon layer 305 from the fiber311. Optical signals not captured by the grating coupler 317 on thefirst pass may be reflected back to the grating by the metal reflector501. Thus, higher coupling efficiencies, as indicated by the thickerarrow for the lateral optical signal in the grating coupler, may beobtained for the grating coupler 317 by integrating the etchedsubstrate/silicon dioxide refill layer and the metal reflector 501. Bycoupling optical input and output structures, such as optical fibers andlight source assemblies, on the CMOS chip back surface 137B, greaterflexibility is enabled for electrical and thermal interfaces on the CMOSchip front surface 137A. For example, the CMOS chip 130 may then beflip-chip bonded to IC packages or other chips via the CMOS chip frontsurface 137A.

FIG. 6 is a schematic of an exemplary optical fiber coupled to the backsurface of a CMOS photonic chip flip-chip bonded to a packagingsubstrate, in accordance with an embodiment of the invention. Referringto FIG. 6, there is shown the CMOS chip 130 comprising the siliconsubstrate 301, the buried oxide 303, the silicon layer 305, thedielectric layers 307, the metal pads 309, the optical fiber 311, epoxy313, the anti-reflection coating 315, the grating coupler 317, thesilicon dioxide refill layer 401, and the metal reflector 501. There isalso shown a packaging substrate 601, flip-chip bump bonds 603, the CMOSchip front surface 137A, and the CMOS chip back surface 137B.

The CMOS chip 130 may be as configured in FIG. 5, but flip-chip bondedto the packaging substrate 601, which may comprise a ceramic material,for example, for housing and/or supporting the CMOS chip 130. Thepackaging substrate 601 may provide a thermal sink for the CMOS chip 130in addition to mechanical support.

In an embodiment of the invention, a method and system are disclosed forcoupling optical signals into silicon optoelectronic chips. Aspects ofthe invention may comprise coupling one or more optical signals into aback surface of a CMOS photonic chip 130 comprising photonic,electronic, and optoelectronic devices 103A-103K, 105A-105D, 107A-107D,111A-111D, 112A-112D, 113A-113H, 115A-115D, 117A-117H, 131, 133, 135,207, 317. The photonic, electronic, and optoelectronic devices103A-103K, 105A-105D, 107A-107D, 111A-111D, 112A-112D, 113A-113H,115A-115D, 117A-117H, 131, 133, 135, 207, 317 may be integrated in afront surface 137A of the CMOS photonic chip 130 and one or more gratingcouplers 135, 139, 207, 317 may receive the one or more optical signalsin the front surface of the CMOS photonic chip 137A. The one or moreoptical signals may be coupled into the back surface 137B of the CMOSphotonic chip 130 via one or more optical fibers 145, 311, which may beaffixed to the CMOS photonic chip 130 via epoxy 313. The one or moreoptical signals may be coupled into the back surface 137B of the CMOSphotonic chip 130 via one or more optical source assemblies 147, whichmay be affixed to the CMOS photonic chip 130 via epoxy 313. The opticalsignals may be coupled to the one or more grating couplers 135, 139,207, 317 via a light path etched in the CMOS photonic chip 130. Theetched light path may be refilled with silicon dioxide 401. The CMOSphotonic chip 130 may be flip-chip bonded to a packaging substrate 601.Optical signals that pass through the one or more grating couplers 135,139, 207, 317 may be reflected back to the one or more grating couplers135, 139, 207, 317 via one or more metal reflectors 501, which may beintegrated in dielectric layers 307 on the CMOS photonic chip 130.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiments disclosed, but that the present inventionwill include all embodiments falling within the scope of the appendedclaims.

1-20. (canceled)
 21. A method for processing signals, the methodcomprising: in a photonic transceiver comprising a silicon photonicchip, coupling one or more optical signals into a back surface of saidsilicon photonic chip through a light path in a region where silicon isremoved from said silicon photonic chip, wherein photonic devices areintegrated in layers on a front surface of said silicon photonic chipand said one or more optical signals are received by one or more opticalcouplers integrated in said layers on said front surface of said siliconphotonic chip.
 22. The method according to claim 21, comprising couplingsaid one or more optical signals into said back surface of said siliconphotonic chip via one or more optical fibers.
 23. The method accordingto claim 21, wherein said optical couplers comprise grating couplers.24. The method according to claim 21, comprising coupling an opticalsource signal for said photonic transceiver into said back surface ofsaid silicon photonic chip via an optical source assembly bonded to saidsilicon photonic chip.
 25. The method according to claim 21, comprisingcoupling said one or more optical signals to said one or more gratingcouplers via an anti-reflective coating on said back surface of saidsilicon photonic chip.
 26. The method according to claim 25, whereinsaid region where silicon is removed from said silicon photonic chipcomprises silicon dioxide.
 27. The method according to claim 21, whereinsaid front surface of said silicon photonic chip is bonded to a secondchip.
 28. The method according to claim 21, comprising reflectingoptical signals that pass through said one or more optical couplers backto said one or more optical couplers via one or more metal reflectors.29. The method according to claim 28, wherein said one or more metalreflectors are integrated in dielectric layers on said silicon photonicchip.
 30. A system for processing signals, the system comprising: aphotonic transceiver comprising a silicon photonic chip with photonicdevices integrated in layers on a front surface of said silicon photonicchip and a light path in a region where silicon is removed from a backsurface of said silicon photonic chip, wherein one or more opticalsignals are coupled into said back surface of said silicon photonic chipvia said light path, and one or more optical couplers integrated in saidlayers on said front surface of said silicon photonic chip are operableto receive said one or more optical signals.
 31. The system according toclaim 30, wherein said one or more optical signals are coupled into saidback surface of said silicon photonic chip via one or more opticalfibers coupled to said light path.
 32. The system according to claim 31,wherein said light path passes through a buried oxide layer in saidsilicon photonic chip.
 33. The system according to claim 30, whereinsaid optical couplers comprise grating couplers.
 34. The systemaccording to claim 30, wherein an optical source signal is coupled intosaid back surface of said CMOS photonic chip via an optical sourceassembly.
 35. The system according to claim 30, wherein said one or moreoptical signals is coupled to said optical couplers through ananti-reflection coating on said back surface of said silicon photonicchip.
 36. The system according to claim 35, wherein said region wheresilicon is removed from said silicon photonic chip comprises silicondioxide.
 37. The system according to claim 30, wherein said frontsurface of said silicon photonic chip is bonded to a second chip. 38.The system according to claim 30, wherein said photonic devices areoperable to reflect optical signals that pass through said one or moregrating couplers back to said one or more grating couplers via one ormore metal reflectors.
 39. The system according to claim 38, whereinsaid one or more metal reflectors are integrated in dielectric layers onsaid silicon photonic chip.
 40. A system for processing signals, thesystem comprising: a photonic transceiver comprising a silicon photonicchip with photonic devices integrated beneath a stack of dielectriclayers in a front surface of said silicon photonic chip, wherein saidfront surface of said silicon photonic chip is bonded to a second chip;and wherein one or more optical couplers integrated beneath a metalreflector embedded in said dielectric layers receive one or more opticalsignals coupled into a back surface of said silicon photonic chipthrough a region where silicon is removed from said silicon photonicschip.